Chemistry - Why do bubbles form when a reactive metal is placed in a less reactive metal's salt solution?

Solution 1:

The bubbles are still hydrogen. Transition metal salts are generally much weaker acids than whatever you put your magnesium ribbon into for your hydrogen control. So you get less gas for the candle flame to react with when you use the transition metal salt, which I am sure you saw. That the decreased amount of gas still popped, which is basically a low-intensity bang, should be deemed a positive test for hydrogen.

Solution 2:

When you dip magnesium ribbon into a copper sulfate solution, theoretically (and realistically) you do get a simple replacement reaction. Then reality sets in: you have a metallic anode (magnesium) with little cathodes (copper) all over it. At that point, the magnesium just overreacts. Well, it reacts faster.

The size of the bubbles will cause some difference in the popping ability of the bubbles. Bubbles could pop differently if some were pure H2 and others were mixed with air.